That last bit isn't true at all for South Korea. Park Chung-hee, the general who ruled for nearly 20 years, was president under 3 constitutions, each further consolidating his power, importantly with the appearance of a democratic mandate. Despite the appearance of a democracy and the Reagan-like legacy he holds, Park was definitely a dictator: under the Yusin constitution, the president (only Park was eligible to run for president per carefully designed requirements) appointed the parliament, could suspend any freedom, and even legislate without his parliament's rubber stamp in emergencies.

Most enterprise was limited to Park's inner circle (which became the chaebol), allowing him to have a planned economy that finally surpassed North Korea's. Labor rights were nonexistent, journalists and other writers frequently became political prisoners, and travel outside the country was not allowed. The only thing that can be convincingly said to be better (holding Western liberal democracy as best) than in North Korea is the GDP, whether that meant much to the average citizen is up for debate since North Korea remains opaque. And it surely would have carried on if not for Park's assassination, which left no obvious successor. The new constitution (remember, Yusin made sure that only Park Chung-hee could be president) had a term limit on the new president, Chun Doo-hwan, to appease democratization activists who had been incensed by the Gwangju (5/18) massacre, the murder of hundreds or thousands of people for the crime of living in a restive province. When Chun's term ended, violent protests, this time in Seoul, forced his successor to make the 6/29 Declaration, a complete capitulation to the democratization activists.

That was long...point is, South Korea's military dictators did claim a democratic mandate, justifying oppression as the only way to economic prosperity (sound familiar?). After Park Chung-hee, they sought to appease democratization activists. Despite that effort, the transition was quite messy, not "fairly smooth": between 200 and 2000 protestors were killed in one week in 1980, and democracy was finally won through violent protests in the streets of Seoul in 1987.

Surely they both justified their military dictatorship as a bulwark from the menace of communism and for liberation/unification of homeland, which sounds to me like "good of the Chinese/Korean people."

I patiently await for you to point me to a cite that documents that the South Korean/Taiwanese military justified their rule because they asserted the right to dress up in military uniforms and lord it over the rest of the population.